Pasquill-Gifford stability classes

TABLE 18.5 Relationship between Pasquill-Gifford Stability Classes and Temperature Stratification... 

TABLE 2.8. Metero/ogical Conditions Defining the Pasquill-Gifford Stability Classes (Gifford, 1976)... 

Description Time and weather Wind Speed, u Monin-Obukhov length, . Pasquill-Gifford stability class... 

TABLE 26-28 Atmospheric Stability Classes for Use with the Pasquill-Gifford Dispersion Model... 

As an example of the use of the Gaussian plume equations using the Pasquill-Gifford dispersion parameters, assume that a source releases 0.37 g s of a pollutant at an effective height of 40 m into the atmosphere with the wind blowing at 2 m s . What is the approximate distance of the maximum concentration, and what is the concentration at this point if the atmosphere is appropriately represented by Pasquill stability class B ... 

In the calculations that were made to predict ground level concentrations from a VCM reactor blow off, the Pasquill-Gifford-Holland dispersion model was used as a basis for these estimations. Calculations were made for six different stability classes and ground level concentrations, and at various distances from the point source of emission. 

From Figures 2 and 3, the Pasquill-Gifford dispersion coefficients are obtained for a downwind distance of 2000 meters and for atmospheric stability Class B. 

The most widely used ay and ct- correlations based on the Pasquill stability classes have been those developed by Gifford (1961). The correlations, commonly referred to as the Pasquill-Gifford curves, appear in Figures 18.4 and 18.5. 

FIGURE 18.4 Correlations for crv based on the Pasquill stability classes A to F (Gifford 1961). These are the so-called Pasquill-Gifford curves. 

Pasquill-Gifford atmospheric stability classes are calculated using the standard deviations of the horizontal wind direction. The stability stratification for the TA-V tower is given using the three-year database. Table 1.4-2 identifies the frequency of stability and wind speed conditions that could be expected in general at TA-V. 

Turner has taken the Pasquill-Gifford work and prodnced a stability classification that is based on honriy meteorological observations taken at weather bnreau stations. This stability classificationhasbeenmade completely objective so that an electronic compnter canbe nsed to compute stability. There are seven stability classes ... 

To make an estimate of oy, or the stabiUty class must first be determined. The two typing techniques of Pasquill-Gifford and Turner discussed previously can be used. Then a series of curves or formulas are referenced to find values for Oy and as a function of stability class, downwind distance, and averaging time. For the values of Ty and 7 that follow, averaging time should be considered to be 1 hr. 

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